A bipartisan group of senators sent a letter to Homeland Security asking for details about the cellular activity. The surveillance activity is akin to so-called stingrays that are used by law enforcement agencies to trick suspects’ cellphones into revealing their locations, according to Reuters.

Last month, the Department of Homeland Security revealed that it had “observed anomalous activity in the National Capital Region that appears to be consistent with International Mobile Subscriber Identity catchers.” It claimed not to know who was behind the use of the surveillance devices….